SU-GG-J-54: Comparison of Head and Neck Localization with Portal Imaging and Megavoltage Cone-Beam Computed Tomography

Purpose: To compare portal imaging (PI) and mega‐voltage cone‐beam CT (MV‐CBCT) for head and neck (HN) cancer patient localization. Method and Materials: The treatment couch shifts were recorded for 30 HN cancer patients localized weekly using anterior‐posterior and right‐lateral portal images, and for 11 patients localized using MV‐CBCT, either daily (n=4) or twice weekly (n=7). A total of 214 and 184 shifts were obtained for PI and MV‐CBCT, respectively, in the left‐right (LR), superior‐inferior (SI) and anterior‐posterior (AP) directions, along with the magnitude of the 3D shifts. The percentage of shifts that are equal to 0 in any direction, that is no treatment couch correction was applied in that direction, was also compared for the two techniques. Results: The average MV‐CBCT and PI shifts were −0.9±3.9 and 0.0±2.2 mm, respectively, in the LR direction, −0.2±2.4 and −0.1±3.5 mm in the SI direction, −1.0±4.2 and −0.2±2.6 mm in the AP direction. The average 3D shifts were 5.2±3.6 mm and 2.2±4.3 mm for MV‐CBCT and PI, respectively. The proportion of 0 mm shifts for MV‐CBCT and PI was 55% and 88%, respectively, in the LR direction, 71% and 81% in the SI direction, 49% and 90% in the AP direction. Conclusion: The three‐dimensional nature of the MV‐CBCT localization method offers a greater ability to detect small shifts and helps reach a compromise between the 3 translational treatment couch shifts that can be applied and the 12 degrees of freedom corresponding to translational and rotational movements of the head and/or neck, independently. This greater accuracy of MV‐CBCT allows to reduce CTV‐to‐PTV margins, therefore potentially improving the therapeutic ratio. Research partially supported by Siemens Medical Solutions.